1. Field of Invention
This invention relates to improvements in methods and apparatuses for dynamic information storage or retrieval, and more particularly to improvements in methods and circuitry used in mass data storage devices, and still more particularly to methods and circuits to retract a carriage assembly of the type used to carry a data transducer, or the like, from a spinning data-containing media disk to a rest or retracted position.
2. Relevant Background
Mass data storage devices include well-known hard disk drive assemblies (HDAs). Generally, an HDA includes one or more rotating disks that carry a magnetic or optical media onto which data may be written, and from which previously written data may be read. The data is written to and read from the disk by one or more heads or transducers that are carried on a selectively movable carriage. The carriage assembly is typically moved and positioned by a voice coil motor (VCM), which moves the heads to the desired locations at which data is to be written or read.
Hard disk drives may be used in many applications, including personal computers, set top boxes, video and television applications, audio applications, or some mix thereof. Applications for hard disk drives still being developed, and are expected to further increase significantly in the future.
More particularly, mass data storage devices typically include either magnetic or optical disks for containing the data. The magnetic disk types generally operate by orienting magnetic domains contained on the disk to predetermined positions in dependence upon the data to be established and detected by signals delivered to and from the head. On the other hand, optical disks usually operate by locally varying the optical properties of the disk to provide a reflectivity gradient that can be detected by a laser transducer head, or the like. Both type disks may be used, for example, to contain data, video, music, or other information.
Details of the construction of a typical HDA are shown in copending U.S. patent application Ser. No. 09/464,315, filed Dec. 16, 1999, entitled METHOD AND CIRCUIT FOR OPERATING A VOICE COIL ACTUATOR OF A MASS DATA STORAGE DEVICE, said patent application being assigned to the assignee hereof and incorporated herein by reference. As shown therein, a typical HDA includes a VCM arranged to move one or more arms (sometimes referred to herein as a “carriage” or “carriage assembly”) in conjunction with a plurality of rotating disks. The arms may be pivoted about a bearing point to carry and move the heads or data transducers radially inwardly and outwardly within the stack of disks to be enabled to selectively read or write data to the disk media.
The outboard end of the arms carries a coil that is selectively energized by currents from a VCM positioning driver. The coil is located between two magnets, which are spaced apart to allow the arm and coil portions to freely swing therebetween. Thus, as currents from the VCM positioning driver are applied to the coil, magnetic fields are established by the current induced field of coil, which interacts with the fields of magnets to precisely position the heads at desired locations.
In operation, as long as there is relative movement between the disk and head, the head flies above the surface of the disk without actually contacting it. When, however, the disk stops, for example, when the power is removed, the air bearing between the disk and head no longer exists, and the head may fall into contact with the disk, resulting in possible damage to both. Thus, as known, a number of situations exist in which it is desired to move the carriage to a position at which the carriage is “parked” or “landed” so that the heads do not contact the disk. Although one preferred location at which the parked position may be defined is adjacent the outer periphery of the disk, it may be parked at other locations as well, for example, adjacent the inner radius of the disk.
In order to properly move the carriage to the park position in contact with the “crash stops” of the park platform, generally a driving current of some particular pattern is applied to the coil that is of sufficient magnitude to bring the carriage to the park position. However, it will be appreciated that if the carriage is overdriven, the delicate head mechanism and other parts of the carriage may sustain damage. On other hand, if the carriage is underdriven, the head mechanism may not reach the park position, which may result in loss of the air bearing between the head and disk surface, which may also cause damage both to the head mechanism and to the underlying media of the disk assembly above which the heads fly.
A typical retract circuit may incorporate a capacitor powered circuit that stores power that is modulated and applied to the VCM in the event of a power failure. Such pulsed techniques, however, have at least two drawbacks. First, the capacitor must provide sufficient power to adequately drive the carriage to the crash stop in the parked position. If the initial velocity is not properly determined, the capacitor power may not be properly metered, and the carriage may fall short of the target position. Secondly, pulsing the power to the VCM results in acoustic noise, which is undesirable in many instances. Depending on the frequency of the pulsing, the pulsing in itself does not necessarily create noise. Frequencies higher than 20 kHz are inaudible to humans. However, one of the major drawbacks of the capacitor solution is that a capacitor will have to be very large to effectively retract an arm with a lot of inertia. This would be expensive. Additionally, the capacitor solution is generally such that the velocity is not well regulated. This is not popular among drive designers because of the fear that the arms will rattle and damage the disk surface if the heads strike something too hard. There is also the concern that the arm will not be retracted to the landing zone.
Another technique that has been used is a two-stage voltage application in which a first, low voltage is applied to the VCM to move the heads to the base of the landing structure, then a large voltage is applied to the VCM to drive the carriage definitively to the crash-stop of the landing structure. The power source for such technique may be, for example, the BEMF of the spindle motor used to spin the data disk. However, in such techniques, the carriage velocity is a function of physical motor parameters like friction, coil resistance, and torque, which causes the carriage velocity to be inaccurate and unreliable.
Still a third technique which has been used is a sample and drive carriage velocity control scheme. In this scheme the coil of the carriage assembly is floated and the velocity inferred by measuring the BEMF of the coil. The voltage level of the BEMF is directly proportional to the angular velocity of the motor. After the measurement of BEMF, the carriage is driven as needed to force a commanded carriage voltage. This implementation, however, does not allow for sophisticated velocity detection. More sophisticated velocity detection could reduce acoustic emissions.
What is needed, therefore, is a method and circuit for reliably moving the head carriage assembly to a parked position, while generating a minimum amount of acoustic noise, and accurately controlling the carriage velocity.